Extend source readme.

Author: drewnoakes

Dasher/README.md

@@ -7,11 +7,15 @@ Primary public types are:
 * `Serialiser` and `Serialiser<T>`
 * `Deserialiser` and `Deserialiser<T>`
 * `DasherContext`
+* `SerialisationException`
 * `DeserialisationException`
 * `UnexpectedFieldBehaviour`
+* `Union<...>`
 
 In addition, some internal types are used within the library:
 
 * `SerialiserEmitter`
 * `DeserialiserEmitter`
 * `ILGeneratorExtensions`
+
+See also the `TypeProviders` sub-namespace.